In recent years, wireless communication devices such as mobile phones have been reduced in size. Thus, the reduction of the number of parts used in the wireless communication device has been considered. For example, to reduce the number of parts, the omission of an interstage filter in a transmit path or a receive path has been considered. In this case, however, it is required to improve the isolation characteristic of a multiplexer. For example, it has been suggested to connect a resonator in parallel to at least one of a transmit filter and a receive filter to improve the isolation characteristic as disclosed in, for example, Japanese Patent Application Publication Nos. 2013-118611 and 2014-82700 (Patent Documents 1 and 2).
When the transmit filter or the receive filter is formed of piezoelectric thin film resonators, according to Patent Document 1, a longitudinally coupled acoustic wave filter is connected in parallel to a filter formed of piezoelectric thin film resonators. In this case, the longitudinally coupled acoustic wave filter is connected in parallel to a filter formed of piezoelectric thin film resonators, and thus is considered to be formed of piezoelectric thin film resonators. However, it is difficult to improve the isolation characteristic in the longitudinally coupled acoustic wave filter formed of piezoelectric thin film resonators. This is because the control of the resonant frequency of the longitudinally coupled acoustic wave filter allows to improve the isolation characteristic but it is difficult to control the resonant frequency of the longitudinally coupled acoustic wave filter formed of piezoelectric thin film resonators.